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Abstract:

A floating heat sink device is provided that attaches to a cage in a
floating configuration that enables the heat sink device to move, or
"float", as the parallel optical communications device secured to the
cage moves relative to the cage. Because the heat sink device floats with
movement of the parallel optical communications device, at least one
surface of the parallel optical communications device maintains
continuous contact with at least one surface of the heat sink device at
all times. Ensuring that these surfaces are maintained in continuous
contact at all times ensures that heat produced by the parallel optical
communications device will be transferred into and absorbed by the
floating heat sink device.

Claims:

1. An optical transceiver module comprising: an optical transceiver
module housing; and an electromagnetic interference (EMI) cancelation
device disposed in the housing, the EMI cancelation device including a
metal chamber having at least first and second walls that are generally
parallel to each other, the first and second chamber walls having first
and second openings formed therein, respectively, through which at least
one optical fiber passes, the first and second chamber walls being
separated from each other by a preselected distance, wherein the
preselected distance is selected to ensure that EMI that passes through
the second opening and is incident on the first wall at a first instant
in time is reflected back toward the second wall and destructively
interferes with EMI passing through the second opening at a second
instant in time that is later than the first instant in time.

2. The optical transceiver module of claim 1, wherein the preselected
distance is equal to, or approximately equal to, one quarter wavelength
of the EMI.

3. The optical transceiver module of claim 2, further comprising: an EMI
absorption material disposed in the metal chamber.

5. The optical transceiver module of claim 3, wherein the EMI cancelation
device is disposed in a nose of the optical transceiver module housing
near a location on the module at which an end of an optical fiber cable
comprising said at least one optical fiber is secured to the housing.

6. A method for attenuating electromagnetic interference (EMI) in an
optical transceiver module, the method comprising: providing an optical
transceiver module having a housing in which a metal chamber is disposed,
the metal chamber having first and second walls that are generally
parallel to each other, the first and second walls having first and
second openings formed therein, respectively, through which at least one
optical fiber extends, wherein the first and second chamber walls are
separated from each other by a preselected distance; and at a first
instant in time, reflecting EMI off of the first wall of the metal
chamber toward the second opening formed in the second wall of the
chamber such that EMI propagating through the second opening at a second
instant in time that is later than the first instant in time
destructively interferes with the EMI reflected off of the first wall.

7. The method of claim 6, wherein the preselected distance is equal to,
or approximately equal to, one quarter wavelength of the EMI.

8. The method of claim 7, wherein the chamber has an EMI absorption
material disposed therein.

10. The method of claim 8, wherein the metal chamber is disposed in a
nose of the optical transceiver module housing near a location on the
module at which an end of an optical fiber cable comprising said at least
one optical fiber is secured to the housing.

Description:

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to optical transceiver modules. More
particularly, the invention relates to a device and method for use in an
optical transceiver module for canceling electromagnetic interference
(EMI).

BACKGROUND OF THE INVENTION

[0002] An optical transceiver module is an optical communications device
that has a transmit (TX) portion and a receive (RX) portion. The TX
portion includes a laser driver circuit and at least one laser diode. The
laser driver circuit outputs an electrical drive signal to each
respective laser diode to cause the respective laser diode to be
modulated. When the laser diode is modulated, it outputs optical signals
that have power levels corresponding to logic 1s and logic 0s. An optics
system of the optical transceiver module focuses the optical signals
produced by each respective laser diode into the end of a respective
transmit optical fiber held within an optical connector module that
connects to the optical transceiver module.

[0003] The RX portion of the optical transceiver module includes at least
one receive photodiode that receives an incoming optical signal output
from the end of a respective receive optical fiber held in the optical
connector module. The optics system of the transceiver module focuses the
light that is output from the end of each receive optical fiber onto the
respective receive photodiode. The respective receive photodiode converts
the incoming optical signal into an electrical analog signal. An
electrical detection circuit, such as a transimpedance amplifier (TIA),
receives the electrical signal produced by the receive photodiode and
outputs a corresponding amplified electrical signal, which is processed
by other circuitry of the RX portion to recover the data.

[0004] Some optical transceiver modules have a single laser diode in the
TX portion and a single photodiode in the RX portion for simultaneously
transmitting and receiving optical signals over transmit and receive
fibers, respectively, of transmit and receive optical cables,
respectively. The ends of the transmit and receive cables have optical
connector modules on them that are adapted to plug into transmit and
receive receptacles, respectively, formed in the optical transceiver
module. These types of optical transceiver modules are often referred to
as pluggable transceiver modules. Small form-factor pluggable (SFP) and
SFP+ transceiver modules are examples of pluggable optical transceiver
modules. Parallel optical transceiver modules of the type described above
may also be configured as pluggable optical transceiver modules, but may
also be configured as mid-plane mounted optical transceiver modules that
mount to a surface of a circuit board.

[0005] Some optical transceiver modules have multiple laser diodes in the
TX portion and multiple photodiodes in the RX portion for simultaneously
transmitting and receiving multiple optical signals. In these types of
optical transceiver modules, which are commonly referred to as parallel
optical transceiver modules, the transmit fiber cables and the receive
fiber cables have multiple transmit optical fibers and multiple receive
optical fibers, respectively. The cables are typically ribbon cables
having ends that are terminated in an optical connector module that is
configured to be plugged into a receptacle of the optical transceiver
module.

[0006] Typically, pluggable optical transceiver modules, such as the SFP
and SFP+ optical transceiver modules, for example, are designed to be
inserted into cages. The pluggable transceiver modules and the cages have
locking features disposed on them that allow the transceiver modules to
mate with and interlock with the cages. The pluggable transceiver modules
typically include latch lock pins that are designed to be received in
latch openings formed in the cages. In most pluggable optical transceiver
module designs, the area around the latch lock pin constitutes an EMI
open aperture that allows EMI to escape from the transceiver module
housing. The Federal Communications Commission (FCC) has set standards
that limit the amount of electromagnetic radiation that may emanate from
unintended sources. For this reason, a variety of techniques and designs
are used to shield EMI open apertures in transceiver module housings in
order to limit the amount of EMI that passes through the apertures.
Various metal shielding designs and resins that contain metallic material
have been used to cover areas from which EMI may escape from the
housings. So far, such techniques and designs have had only limited
success, especially with respect to optical transceiver modules that
transmit and receive data at very high data rates (e.g., 10 gigabits per
second (Gbps)).

[0007] For example, EMI collars are often used with pluggable optical
transceiver modules to provide EMI shielding. The EMI collars in use
today vary in construction, but generally include a band portion that is
secured about the exterior of the transceiver module housing and spring
fingers having proximal ends that attach to the band portion and distal
ends that extend away from the proximal ends. The spring fingers are
periodically spaced about the collar. The spring fingers have folds in
them near their distal ends that cause the distal ends to be directed
inwards toward the transceiver module housing and come into contact with
the housing at periodically-spaced points on the housing. At the
locations where the folds occur near the distal ends of the spring
fingers, the outer surfaces of the spring fingers are in contact with the
inner surface of the cage at periodically spaced contact points along the
inner surface of the cage.

[0008] The amount of EMI that passes through an EMI shielding device is
proportional to the largest dimension of the largest EMI open aperture of
the EMI shielding device. Therefore, EMI shielding devices such as EMI
collars and other devices are designed to ensure that there is no open
aperture that has a dimension that exceeds the maximum allowable EMI open
aperture dimension associated with the frequency of interest. For
example, in the known EMI collars of the type described above, the
spacing between the locations at which the distal ends of the spring
fingers come into contact with the inner surface of the cage should not
exceed one quarter wavelength of the frequency of interest that is being
attenuated. Even greater attenuation of the frequency of interest can be
achieved by making the maximum EMI open aperture dimension significantly
less than one quarter of a wavelength, such as, for example, one eighth
or one tenth of a wavelength. However, the ability to decrease this
spacing using currently available manufacturing techniques is limited. In
addition, as the frequency of optical transceiver modules increases, this
spacing needs to be made smaller in order to effectively shield EMI,
which becomes increasingly difficult or impossible to achieve at very
high frequencies.

[0009] In parallel optical transceiver modules, the optical cables that
carry the fibers are typically ribbon cables in which the fibers are
arranged side-by-side in a 1×N array, where N is the number of
fibers of the ribbon cable. Thus, the transmit fibers are arranged in one
1×N fiber array in one ribbon cable and the receive fibers are
arranged in another 1×N array in another ribbon cable. Typically,
the ribbon cables are placed one on top of the other such that a
2×N array of fibers enter the optical connector module through a
gap formed in the nose of the optical connector module. This gap
constitutes an EMI open aperture that is much larger than the maximum
allowable EMI open aperture dimension of the optical transceiver module,
particularly at high bit rates. Consequently, unacceptable amounts of EMI
may escape from the optical transceiver module through the gap.

[0010] One technique that is sometimes used to provide EMI shielding at
the gap in the optical connector module involves placing a metal EMI
shielding device in the nose of the optical connector module surrounding
the gap such that the fibers pass through the EMI shielding device. While
such shielding devices are relatively effective at preventing EMI from
passing through regions in the housing immediately adjacent the gap, they
are totally ineffective at preventing EMI from passing through the gap
itself, which is filled only with the fibers and air. Of course, the
fibers and the air are transparent to EMI.

[0011] In general, all of the current techniques of providing EMI
shielding in optical transceiver modules attempt to ensure that there are
no EMI open apertures that have dimensions that exceed the maximum
allowable EMI open aperture dimension. As indicated above, as the
frequencies or bit rates of optical transceiver modules continue to
increase (i.e., wavelengths continue to decrease), it becomes extremely
difficult or impossible to effectively implement these types of
solutions. Accordingly, a need exists for an EMI shielding device and a
method that do not rely solely on such techniques to provide effective
EMI shielding in optical transceiver modules.

SUMMARY OF THE INVENTION

[0012] The invention is directed to an optical transceiver module having
an EMI cancelation device and an EMI cancelation method. The optical
transceiver module comprises an optical transceiver module housing having
the EMI cancelation device disposed therein. The EMI cancelation device
includes a metal chamber having at least first and second walls that are
generally parallel to each other. The first and second chamber walls have
first and second openings formed therein, respectively, through which at
least one optical fiber passes. The first and second chamber walls are
separated from each other by a preselected distance selected to ensure
that EMI that passes through the second opening and is incident on the
first wall at a first instant in time is reflected by the first wall back
toward the second wall and destructively interferes with EMI passing
through the second opening at a second instant in time that is later than
the first instant in time.

[0013] The method comprises providing an optical transceiver module having
a housing in which a metal chamber is disposed, and, at a first instant
in time, reflecting EMI off of a first wall of the metal chamber toward
an opening formed in a second wall of the chamber such that EMI
propagating through the second opening at a second instant in time that
is later than the first instant in time destructively interferes with the
EMI reflected off of the first wall.

[0014] These and other features and advantages of the invention will
become apparent from the following description, drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 illustrates a perspective side view of a parallel optical
transceiver module that is equipped with an EMI cancelation device in
accordance with the invention.

[0016]FIG. 2 illustrates a side cross-sectional view of the optical
transceiver module shown in FIG. 1.

[0017]FIG. 3 illustrates a plan view of the ends of the optical fibers
that pass through an opening in the optical transceiver module shown in
FIGS. 1 and 2 that constitutes an EMI open aperture.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

[0018] In accordance with the invention, an optical transceiver module is
equipped with an EMI cancelation device having a propagation chamber that
has dimensions selected based on the primary frequency of the optical
transceiver module such that at least some of the EMI propagating in the
chamber experiences destructive interference and is canceled. In
addition, the EMI cancelation device may include an EMI absorbing
material that absorbs at least some EMI that is not canceled.
Consequently, the EMI cancelation device is very effective at attenuating
EMI to prevent it from escaping from the optical transceiver module.
Illustrative, or exemplary, embodiments of the invention will now be
described with reference to the figures.

[0019] FIG. 1 illustrates a perspective view of a parallel optical
transceiver module 1 that incorporates an EMI cancelation device (not
shown), which will be described below in detail with reference to FIG. 2.
In accordance with this illustrative embodiment, the parallel optical
transceiver module 1 is part an active optical cable that includes first
and second optical ribbon cable 6, an optical connector module 10
connected to a boot 8 that is secured to the end 6a of the cable 6, and a
plug 20 mechanically coupled with the optical connector module 10. The
optical connector module 10 and the plug 20 have housings 10a and 20a,
respectively, which are typically made of metal, such as metal die cast
material, for example.

[0020] The plug 20 of the optical transceiver module 1 is configured to be
inserted into an opening formed in a cage (not shown). Locking pins 5
located on the plug housing 20a are received in respective latch openings
formed in the cage when the plug 20 is fully inserted into the cage,
thereby interlocking the plug 20 with the cage. In this interlocked
position, first and second rows of electrical contacts 13 and 14 disposed
on first and second circuit boards 21 and 22, respectively, are received
in respective slots (not shown) of an electrical connector (not shown)
located inside of the cage to make electrical connections between the
optical transceiver module 1 and the electrical connector.

[0021]FIG. 2 illustrates a side cross-sectional view of the parallel
optical transceiver module 1 shown in FIG. 1. The first and second
circuit boards 21 and 22, respectively, of the plug 20 have TX components
and RX components mounted thereon. The TX components include a plurality
of laser diodes 23a and laser diode driver circuitry 23b. The RX
components 24 include a plurality of photodiodes 24a and receiver
circuitry 24b. A first optical coupling system 25 of the plug 20 is
attached to ends (not shown) of transmit optical fibers 11 of the ribbon
cable 6. A second optical coupling system 26 of the plug 20 is attached
to ends (not shown) of receive optical fibers 12 of the ribbon cable 6.
During operations, optical signals produced by the laser diodes 23a are
coupled via the first optical coupling system 25 into the ends of the
transmit optical fibers 11 and optical signals passing out of the ends of
the receive optical fibers 12 are coupled via the second optical coupling
system 26 onto the photodiodes 24a.

[0022] As shown in FIGS. 1 and 2, the optical transceiver module 1
includes an EMI collar 31 of the type described above for providing EMI
shielding. The EMI collar 31 is secured about the periphery of the
housing 20a of the plug 20 near where the locking pins 5 couple with
latch openings formed in the cage (not shown). The EMI collar 31 is
effective at preventing EMI from escaping through the latch openings.
There are other areas, however, where EMI may escape from the optical
transceiver module 1 if other EMI shielding is not employed. In
particular, an opening 35 (FIG. 2) exists in the optical connector module
10 through which EMI may escape. The opening 35 is provided to allow the
optical fibers 11 and 12 to pass through the optical connector module 10
into the plug 20. The ribbon cable 6 carries a 1×N array of the
transmit optical fibers 11 and a 1×N array of the receive optical
fibers 12, where N is the number of fibers in each of the arrays. Thus,
the opening 35 is at least large enough to allow the 2×N array of
optical fibers to pass through it.

[0023]FIG. 3 illustrates a plan view of the ends 11a and 12a of the
optical fibers 11 and 12, respectively, which pass through the opening
35. The opening 35 constitutes an EMI open aperture. The dashed line 36
in FIG. 3 represents the largest dimension of the EMI open aperture
corresponding to the opening 35. For cases in which the primary frequency
of the optical transceiver module 1 is relatively high (e.g., 5 gigahertz
(GHz)=10 Gbps), the EMI open aperture dimension represented by the dashed
line 36 is much larger than the maximum allowable EMI aperture dimension,
which is about one wavelength at the primary frequency. Therefore, if a
proper EMI shielding solution is not employed, an unacceptable amount of
EMI will escape through the opening 35.

[0024] With reference again to FIG. 2, an EMI cancelation device is
disposed in the nose 10b of the optical connector module 10. As indicated
above, the housing 10a of the optical connector module 10 is made of
metal. The EMI cancelation device comprises a metal chamber 110 having at
least first and second walls 110a and 110b, respectively, that are spaced
apart from each other by a distance that is equal to, or substantially
equal to, one-quarter wavelength of the primary frequency of the optical
transceiver module 1. The aforementioned opening 35 is formed in the
first chamber wall 110a. A second opening 37 is formed in the second
chamber wall 110b. The fibers 11 and 12 pass through the openings 35 and
37.

[0025] During operations, EMI of the primary frequency that is generated
within the plug 20 propagates through the second opening 37 formed in the
second chamber wall 110b and into the metal chamber 110. At least a
portion of the EMI that enters the metal chamber 110 is incident on the
first chamber wall 110a and reflected thereby back toward the second
chamber wall 110b. Because the distance between the chamber walls 110a
and 110b is one-quarter wavelength, the EMI reflected by the first
chamber wall 11a at a first instant in time and the EMI entering the
chamber 110 through the second opening 37 at a second instant in time
that is later than the first instant in time destructively interfere. The
destructive interference causes at least a significant portion of the EMI
to be canceled and thus prevented from escaping through the opening 35.

[0026] The EMI cancelation device attenuates EMI by about 20 decibels
(dB). The amount of EMI that is attenuated by the EMI cancelation device
can be increased by disposing an EMI absorption material 120 in the
chamber 110. The EMI absorption material 120 may be any material that
absorbs EMI of the frequency or frequencies of interest. One type of
material that is suitable for this purpose is ECCORS ORB® material,
which is a material that is sold by Emerson & Cuming Microwave Products
of Belgium. ECCOSORB® material is a polyurethane foam material
impregnated with carbon black dispersions having controlled conductivity.
The inclusion of the EMI absorption material 120 in the chamber 110
further attenuates EMI. The combination of the EMI cancelation effects of
the chamber 110 and the EMI absorption effects of the EMI absorption
material 120 result in an overall EMI attenuation of about 60 dB.

[0027] While the EMI cancelation device is designed to attenuate EMI of
the frequency of interest, the EMI cancelation device also cancels and/or
absorbs EMI at other frequencies. For example, EMI at frequencies that
are harmonics of the frequency of interest will also experience
destructive interference in the chamber 110. In addition, the EMI
absorption material 120 typically is capable of absorbing EMI having a
relatively broad range of frequencies. Therefore, the EMI cancelation
device is effective at attenuating EMI over a range of frequencies
including, but not limited to, the frequency of interest. Also, the
distance between the first and second chamber walls 110a and 110b can be
selected to be less than or greater than a quarter wavelength of the
primary frequency (e.g., one eighth wavelength or one half wavelength) to
detune the chamber 110 such that wavelengths other than, or in addition
to, the primary wavelength are attenuated.

[0028] It should be noted that the invention has been described with
respect to illustrative embodiments for the purpose of describing the
principles and concepts of the invention. The invention is not limited to
these embodiments. As will be understood by those skilled in the art in
view of the description being provided herein, modifications may be made
to the embodiments described herein without deviating from the scope of
the invention. For example, while the invention has been described with
reference to a particular type of optical transceiver module, the
invention is not limited to being used with optical transceiver modules
having any particular configuration. Also, the invention may be used in
optical transmitter modules that do not have receiver functionality and
in optical receiver modules that do not have transmitter functionality.
The term "optical transceiver module", as that term is used herein, is
intended to denote all such modules.